1. Field of the Invention
The invention relates to a clamp for sheet material articles.
2. Description of Prior Art
A clamp of this type is known from DE-U-17 75 847. In this known clamp, the clamping element is formed as a circular disk fixedly connected to a rigid lever arm extending outwards through a slot formed in the wall portion of the clamp defining said guide surface. Outwards of the clamp the lever arm is provided with a knob portion. The lever arm is not connected to the housing of the clamp, but is pivotable about a support provided at the lower end of the slot. As particularly evident from FIG. 1, the lateral boundaries of the clamp extend at a distance from one another substantially corresponding to the width of the circular disk or weight, respectively, so as to prevent lateral movement thereof. The side walls of the housing of the clamp are provided with a slot (mouth opening) adjacent the clamping surface. The width of the slot is substantially smaller than the distance of the guide surface from the clamping surface at the level of the entrance opening.
To initiate clamping, a piece of sheet material is inserted into the mouth opening through the entrance opening so as to enter between the clamping element and the clamping surface. The clamping action for holding the piece of sheet material is exerted exclusively by the weight of the clamping element and the biasing weight of the sheet material. An amplification of the clamping action by vertical pulling forces acting on the sheet material is not intended and obviously scarcely possible. A pulling force exerted on the sheet material would tend to rotate the circular disk, whereby the lever arm would be pivoted towards the upper end of the slot. As a result, the lever arm acts on the biasing weight to lift it out of engagement with the clamping element. As a result, the effect of the biasing weight on the clamping force is considerably reduced. In addition, the clamping element can only be rotated until the lever arm is stopped by the upper end of the slot. Further rotation could only be accomplished by bending the lever arm between the stop at the upper end of the slot and the support, and such bending is not desirable.
The known clamp can only be used in the manner described when the center of gravity of the clamping element lies below the support, which is only the case when clamping relatively thin single layers of a sheet material, and particularly only when the clamp is disposed vertically. Inclination of the clamp causes the clamping action to be automatically released. If a multiple-layer sheet material to be clamped is of an overall thickness causing the lever arm to engage the lower boundary edge of the slot, as shown for instance in FIG. 2 by the phantom lines depicting the release position for single-layer sheet material in which position the lever arm can no longer pivot about the support, the load of the clamping element and the biasing weight is supported on the slot edge so that scarcely any clamping force can be brought to bear. If the thickness of the articles to be clamped increases still further, the clamping action will become unstable because the weight of the knob member, combined with the full length of the lever arm, tends to lift the clamping element off the clamping surface. Even if the knob member is relatively lightweight and the lever arm relatively short, the slightest unintentional contact of these members with another object will be sufficient for releasing the clamping action.
Release of the sheet material from the clamp in any case requires the clamping element to be raised with the aid of the knob member, with the force required for releasing the sheet material increasing in proportion to the strength of the clamping action to be overcome.
DE-A- No. 29 49 175 describes a clamping device for a sheet material designed particularly for use with printed circuit boards. The clamping device comprises a base member formed with a clamping surface and a guide surface disposed opposite one another and converging inwards from the entrance opening. The clamping element is a clamping roller, the axle of which is guided in horizontally extending slots of a U-shaped bracket extending laterally around the base member. The U-shaped bracket contains a counterbearing for one end of a spring, the other end of which is supported on an upper surface of the base member.
In order to insert a printed circuit board into the entrance opening of the base member, the bracket is depressed to thereby compress the spring, permitting the clamping roller to be cammed out of the way in a lateral direction. As the bracket is then released it is pushed upwards by the action of the expanding spring, causing the converging guide surface to displace the clamping element towards the printed circuit board resting against the clamping surface and into a clamping position. In this embodiment of a clamping device, the force of the spring has to be accurately determined with respect to the maximum weight of the printed circuit boards. If the articles to be clamped are too heavy so as to exert an excessive pulling force, the clamping roller may be rotated and move downwards with compression of the spring so as to release the clamped article.
Known from DE-A-No. 29 08 786 is a clamp comprising a loop-shaped bracket of unitary or composite construction, the clamping surface being formed on one leg of the bracket or on a wall to which the bracket is attached. The other leg of the bracket is formed as a guide member and inclined towards the clamping surface, the free end of the guide member cooperating with the clamping surface to define an entrance opening whereat its distance from the clamping surface is smallest.
The clamping element has a guide opening extending therethrough, the cross-sectional shape of which corresponds to that of the guide member to thus permit the clamping element to be slipped onto the guide member. The distance of the guide member from the clamping surface is so selected that the clamping element comes into contact with the clamping surface shortly before arriving at the entrance opening. This implies, however, that the width of the entrance opening has to be noticeably smaller than the radius or the thickness of the clamping element, since the clamping element would otherwise be prevented from coming into contact with the clamping surface.
A sheet material article to be clamped is inserted through the entrance opening into engagement with the clamping element. The clamping element is then displaced along the guide member and away from the entrance opening by manual operation or by the action of the inserted article, so that the sheet material article can proceed to a position between the clamping element and the clamping surface. The clamping element is then returned towards the entrance opening by the action of gravity and/or by the action of a spring, to thereby clampingly retain the article. A pulling force subsequently exerted on the article and tending to pull it out of the entrance opening causes the clamping element to be frictionally entrained so as to clamp the article even more tightly against the clamping surface. As a result, also the intentional release of the clamped article becomes increasingly difficult. In addition, a pulling force exerted on the article will not only tend to displace the clamping element in a radial direction relative to the guide member, but will also tend to rotate the clamping member. This results in the guide opening being tilted relative to the guide member, whereby the intentional release of the article is rendered even more difficult. On the other hand, this tilting leads to increased wear of the guide opening and the guide member, so that the clamping action is impaired. For facilitating the intentional release of a clamped article, it has been proposed to form the guide member and the guide opening of the clamping element, preferably a spherical member, with a circular cross-sectional shape permitting the clamping element to be rotated about the guide member. This construction is intended to facilitate release of the clamped article in lateral directions. This intention neglects the fact, however, that the spherical clamping member is prevented from being rotated when it is clamped or tilted on the guide member in the manner described above.